The present invention relates to a new process and a new apparatus for preparing granulates. The invention also relates to granulates which are obtained by means of the new process.
There already are numerous known processes for preparing granular material by fluidised-bed granulation (compare Chem. Ing. Techn. 45, 736-739 (1973), DE-OS (German Published Specification) 2,231,445, DE-OS (German Published Specification) 2,555,917 and EP-OS (European Published Specification) 0,087,039). In these disclosed processes, which are carried out continuously, the ready-to-use granulate is obtained in one step without separate post-drying. In essence, it is possible to distinguish three different methods in this context which, however, are all based on the same granulate formation process. Thus, in each case the product to be granulated is of sprayable consistency, that is to say in the form of a melt, suspension or solution, when sprayed into a fluidised bed. The sprayed-in liquid product first wets the surface of the solid particles in the fluidised bed and then solidifies by drying on or cooling down. In this way the particles grow in shell-like fashion, and their size increases the longer they dwell in the fluidised bed. For that reason the granulate size depends critically upon the bed content.
The growth of the particles starts in the fluidised bed from nuclei which either are formed in the fluidised bed itself by non-impacting solidifying drops of spray or are produced by abrasion of solid particles already present, or which are fed into the fluidised bed from the outside. Nucleation which takes place in the fluidised bed (internal nucleation) is affected by the content of the fluidised bed in such a way that as the bed content grows there is on the one hand a decrease in the number of non-impacting droplets of spray and on the other an increase in the number of nuclei produced by abrasion.
The process which is required for producing granulates in a predetermined grain size is, in fluidised-bed granulation, the interaction of nucleus availability and granulate growth. Therefore, the process of granulation can be affected in many and varied ways. Thus, for example, the availability of nuclei can be increased by adding nuclei from the outside.
Steady-state granulation conditions have been reached when the bed content remains constant. In the steady state the mass of solid material fed into the fluidised bed must correspond to the mass of completed granulates removed from the bed. In addition to this mass balance, however, the particles need also to be in balance. That is to say, all the granulates removed from the fluidised bed must be replaced in number by new nuclei. In the known methods, a constant granulate formation process is forced on the system. The methods differ in the manner in which this constancy is obtained.
In the case of the processes disclosed in DE-OS (German Published Specification) 2,231,445 and EP-OS (European Published Specification) 0,087,039, only internally formed nuclei are used. The control of the associated low fluidised-bed content actuates a discharge element according to the principle of fill level control and hence ensures that the granulate removal matches product intake. The granulate emerging from the granulator is classified, and the resulting fines are returned into the granulator. Since, however, in the case of different throughputs through the serially connected apparatus elements of classifying line and discharge element the fill level control gets out of step, the only classifying line which can be used in this case is a classifying line where the amount of granular material which is let through matches the required throughput through the discharge element irrespective of separating efficiency. Accordingly, the classifying line employed in the methods described in DE-OS (German Published Specification) 2,231,445 and EP-OS (European Published Specification) 0,087,039 is in each case a second fluidised bed. This second fluidised bed provides only very inefficient classification of the granulates. For that reason, this process is unsuitable for producing granulates having a narrow grain size distribution.
The granulating process communicated in DE-OS (German Published Specification) 2,263,968 is in principle identical to the method described above. However, the classifying line employed in this instance is not a second fluidised bed but an efficiently separating sieve. The necessary synchronisation of the throughputs through the classifying line and the discharge element is achieved by not removing a portion of the granular material but instead grinding it and then returning it to the fluidised bed. This additional availability of nuclei needs to be compensated by reducing nucleation in the fluidised bed. The reduction of nucleation in the fluidised bed is achieved by operating the granulator at a high bed content. However, the prerequisite for this mode of operation is an abrasion-resistant granulate.xe2x80x94This method thus provides granulates of narrow grain size distribution. However, it is a disadvantage that a comparatively high amount of apparatus needs to be used. In addition, this method cannot be used for granulating solvent-moist or dust explosion hazard products, since the apparatus required in addition to the fluidised-bed granulator cannot be inertised nor be installed to withstand explosive pressure.
In the third alternative disclosed in DE-OS (German Published Specification) 2,555,917 for producing granulates by the fluidised-bed/spray method, the discharge element used is a countercurrent gravity classifier This classifier combines the functions of classifying line and discharge element.
Using this apparatus, only granulates which have reached the desired grain size are removed from the fluidised bed. Any variation in the number of discharged granulates has an immediate effect on the bed content. If, for example, the bed content rises, the granulates produced are too small. Consequently, the growth of granulates needs to be promoted and the feed of nuclei, which is in this case from the outside, needs to be cut back. To be able to apply effective control measures to the number of nuclei available for the granulation process it is necessary for internal nucleation to be minimised, which, in the case of abrasion-resistant granulates, can be obtained by granulation with high bed content.xe2x80x94In the final analysis this known method produces granulates within a narrow grain size distribution. However, it is a disadvantage that the method requires controlled outside supply of nuclei which is complicated. Furthermore, conversion of the process from a certain mean grain size to another mean grain size is associated with extensive preliminary experimental work to determine the exact settings. That is also true of the other processes described above.
A new process has now been found for the continuous preparation of granulates having narrow grain size distribution, characterised in that
a) the product to be granulated is sprayed in liquid form into a fluidised bed,
b) the proportions of fine material escaping from the fluidised bed in the off-gas are separated off and returned into the fluidised bed as nuclei for granulate formation,
c) the granulation process in the fluidised bed is influenced solely by setting the classifying gas stream in such a way that granulates are formed which are of the size predetermined by the classifying gas stream, and
d) the completed granulate is removed solely by way of one or several countercurrent gravity classifiers which are inserted into the outflow bottom of the fluidised-bed apparatus, and
e) if desired the granulates thus obtained are subjected to a thermal aftertreatment.
A new apparatus for the continuous preparation of granulates having a narrow grain size distribution has also been found. The apparatus essentially comprises a fluidised-bed reactor
which contains devices for dispersing the product supplied in sprayable form,
which further contains a system suitable for returning proportions of fine material escaping from the fluidised bed and
to the outflow bottom of which there are directly attached one or several countercurrent gravity classifiers.
Finally, new granulates found and prepared by the process according to the invention
contain 1 to 100 per cent by weight of at least one active component, 0 to 99 per cent by weight of inert filler material and 0 to 40 per cent by weight of dispersing and/or binding agent and optionally additives,
have a mean grain size of 0.1 to 3 mm,
have a narrow grain size-distribution wherein the largest and the smallest particle diameters differ from the mean by at most half an average grain size,
are uniformly shaped and homogeneously constituted and have a compact microporous structure and
are spontaneously dispersible or soluble in water or other solvents.
The process according to the invention differs from all the corresponding predisclosed processes in that the granulate formation process, through the interaction between granulate growth and nucleation, automatically adjusts to the size of the discharged granulates which is predetermined by the classifying gas supply.
The process according to the invention is distinguished from the analogous predisclosed methods by a number of advantages. For instance, granulates of whichever particle size is desired can be prepared and the particle diameter (grain spectrum) is within very narrow limits. Furthermore, the size of the particles can be varied from case to case in a simple manner by means of the classifying gas supply; no changes to the apparatus are necessary. On the contrary, the particle size can even be changed without interruption of production.
It is of particular advantage that only granular material, i.e. granulates, of the desired size is produced. No material is lost, since undersize grainxe2x80x94that is to say particles which are too smallxe2x80x94remain in the fluidised bed until they have reached the desired size. Oversize grainxe2x80x94that is to say excessively large particlesxe2x80x94is likewise not formed since the particles are removed from the fluidised bed by constant classifying. Grinding processes and sieving processes are therefore dispensed with completely. Nor is it necessary to add outside nuclei to affect the process. It is also favourable that the liquid products to be sprayed into the fluidised bed in the course of the process according to the invention can have a very high solids content. The resulting granulates are uniformly sha ed and homogeneously constituted and, despite high strength, are spontaneously dispersible or dissolvable in water or other solvents. Since the process makes low demands on the abrasion resistance of the granulates, it is also possible to prepare granulates of low binder content, thereby favouring their dispersing properties. Finally, the process according to the invention can also be used for processing solvent-moist and dust explosion hazard products since the required apparatus can be inertised and be constructed to withstand explosive pressures.
In the process according to the invention, the product to be granulated is sprayed in liquid form into a fluidised bed. This liquid form can be a melt, solution or suspension (slurry).
The liquid to be sprayed in can contain one or more active components. Suitable active components are not only substances which are solid at room temperature but also those which are liquid at room temperature. The only precondition for the use of liquid active components is that they are applied to solid carrier substances before granulation. The active components can be soluble or insoluble in water. They need to be stable to hydrolysis to the extent that, in the course of the process according to the invention and in the course of application of the resulting granulates, they do not undergo noticeable decomposition in the presence of water.
Possible active components are agrochemical active substances, active substances for combating pests in the domestic and hygiene areas, pharmacologically active substances, nutriments, sweeteners, dyestuffs and organic or inorganic chemicals.
Agrochemical substances are to be understood as meaning in the present case active substances which are customarily usable in crop protection. They preferably include insectcides, acaricides, nematicides, fungicides, herbicides, growth regulators and fertilisers. Specific examples of such active substances are: O,O-diethyl O-(4-nitrophenyl) thionophosphate, O,O-dimethyl O-(4-nitrophenyl) thionophosphate, O-ethyl O-(4-methylthiophenyl) S-propyldithiophosphate, (O,O-diethylthionophosphoryl)-xcex1-oxominophenylacetonitrile, 2-isopropoxyphenyl N-methylcarbamate, 3,4-dichloropropionanilide, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-(4-chlorophenyl) 1,1-dimethylurea, N-(2-benzothiazolyl)-N,Nxe2x80x2-dimethylurea, 3-(3-chloro-4-methylphenyl)-1,1-dimethylurea, 3-(4-isopropylphenyl)-1,1-dimethylurea, 4-amino-6-(1,1-dimethylethyl)-3-methylthio-1,2,4-triazin-5(4H)-one, 4-amino-6-(6,1-dimethylethyl)-3-ethylthio-1,2,4-triazin-5(4H)-one, 1-amino-6-ethylthio-3-(2,2-dimethylpropyl)-1,3,5-triazine-2,4-(1H,3H)-dione, 4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one; 2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine, the R-enantiomer of the trimethylsilylmethyl ester of 2-[4-(3,5-dichloropyridyl-2-oxy)-phenoxy]-propionic acid, the R-enantiomer of the 2-benzyloxyethyl ester of 2-[4-(3,5-dichloropyridyl-2-oxy)-phenoxy]-propionic acid, 2,4-dichlorophenoxyacetic acid, 2-(2,4-dichlorophenoxy)-propionic acid, 4-chloro-2-methylphenoxyacetic acid, 2-(2-methyl-3-chlorophenoxy)-propionic acid, 3,5-diiodo-4-hydroxybenzonitrile, 3,5-dibromo-4-hydroxybenzonitrile and diphenyl ethers and phenylpyridazines, such as, for example, pyridates, furthermore 2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate, 3,5-dimethyl-4-methylthiophenyl N-methylcarbamate, O,O-diethyl O-(3-chloro-4-methyl-7-coumarinyl) thiophosphate, N,N-dimethyl-Nxe2x80x2-(fluorodichloromethylmercapto)-Nxe2x80x2-(4-methylphenyl) sulfamide, 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1,2,4-triazol-1-yl)-butan-2-one, 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1,2,4-triazol-1-yl)-butan-2-ol, 1-cyclohexyl-4,4-dimethyl-3-hydroxy-2-(1,2,4-triazol-1-yl)-pent-1-ene, 2-(2-furyl)-benzimidazole, 5-amino-1-bis-(dimethylamido)-phosphoryl-3-phenyl-1,2,4-triazole, 4-hydroxy-3-(1,2,3,4-tetrahydro-1-naphthyl)-coumarin, O,O-dimethyl S-[1,2-bis-(ethoxycarbonyl)-ethyl]-dithiophosphate, O,O-dimethyl O-(4-methylmercapto-3-methylphenyl)-thionophosphate, O-ethyl O-(2-isopropyloxycarbonylphenyl) N-isopropylamidothionophosphate and (S)-xcex1-cyano-3-phenoxybenzyl (1R)-cis-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylate.
Active substances for combating pests in the domestic and hygiene are as are to be understood as meaning in the present case substances which can customarily be used for such purposes. Examples which may be mentioned are: 2-isopropoxyphenyl-N methylcarbamate, O,O-diethyl O-(4-nitrophenyl) thionophosphate, O,O-dimethyl O-(4-nitrophenyl) thionophosphate, O,O-dimethyl S-[1,2-bis-(ethoxycarbonyl)-ethyl]-dithiophosphate, O,O-dimethyl O-(3-methyl-4-nitrophenyl) thionophosphate, O,O-dimethyl O-(4-methylmercapto-3-methylphenyl) thionophosphate, cyclohex-1-ene-1,2-dicarboximidomethyl 2,2-dimethyl-3-(2-methylpropenyl)-cyclopropanecarboxylate.
Pharmacologically active substances is to be understood as meaning in the present case not only substances which can be used in veterinary medicine but also substances which can be used in human medicine. An example which may be mentioned of a substance which can be used in veterinary medicine is xcex1-cyano-3-phenoxy-4-fluorobenzyl 2,2-dimethyl-3-[xcex2-(p-chlorophenyl)-xcex2-chlorovinyl]-cyclopropanecarboxylate. An example which may be mentioned of a substance which can be used in human medicine is acetylsalicylic acid.
Nutriments is to be understood as meaning not only substances for human consumption but also substances for animal consumption. Examples which may be mentioned are: citric acid, vitamins, coffee powder, tea powder and cocoa powder.
Examples which may be mentioned of sweeteners are sodium cyclamate and saccharin.
Dyestuffs is to be understood as meaning in the present case substances suitable for preparing dyestuff dispersions or dyestuff solutions which are used as colorants and/or paints. For instance, water-soluble dyestuffs, such as anionic, cationic and reactive dyestuffs, or water-insoluble dyestuffs, such as vat dyestuffs, polyester dyestuffs and pigment dyestuffs, can be used. Examples which may be mentioned are: indanthrene dyestuffs, cerofix dyestuffs, astrazone dyestuffs, triarylamine dyestuffs, triarylmethane dyestuffs, methine dyestuffs, anthraquinone dyestuffs, indigo dyestuffs, sulphur dyestuffs, azo dyestuffs and pigment dyestuffs.
Suitable organic and inorganic chemicals are those substances which are preferably used in the form of aqueous dispersions for synthetic purposes. It is also possible to use aqueous zeolite suspensions. Zeolites is to be understood as meaning substances of this type as described in Ullmann, 4th edition, volume 17, pages 9 et seq. under the entry xe2x80x9cMolekularsiebexe2x80x9d [Molecular sieves].xe2x80x94It is also possible to use suspensions of inorganic oxides which are suitable for preparing catalysts or catalyst supports. Examples which may be mentioned thereof are aluminium oxide and silicon dioxide.
The liquid product to be sprayed into the fluidised bed in the course of the process according to the invention, in addition to the active components and to any liquid diluent present, can also contain inert fillers, dispersants, binders and/or additives, such as, for example, preservatives and dyestuffs.
The fillers can be any filler and carrier substance customarily used in water-dispersible or water-soluble granulates or in oil- and oil-soluble granulates. Substances of that type which are preferably used are inorganic salts, such as alkali metal, magnesium and ammonium chlorides and sulphates, for example magnesium sulphate, potassium sulphate, sodium sulphate, potassium chloride, ammonium sulphate, lithium sulphate and ammonium chloride, furthermore oxides, such as magnesium oxide, nitrates, carbonates, hydrogen carbonates, silicates, talc, chalk, quartz powder, kaolin, montmorillonite, bentonite, attapulgite and sepiolite, and also graphite, furthermore urea and urea derivatives, such as hexamethylenetetramine and casein, and also carbohydrates, such as starch, sugar, alginates and derivatives thereof, cereal flours, such as wheat flour and rice flour, and also kelzanes, methylcellulose and hydroxypropylmethylcellulose, and finally water-soluble polymers, such as polyvinyl alcohol and polyvinylpyrrolidone.
The dispersants used are preferably: condensation products of aromatic sulphonic acids and formaldehyde, such as condensation products of sulphonated ditolyl ether and formaldehyde, and also ligninsulphonic acid salts, such as lithium, sodium, potassium, magnesium, calcium and ammonium salts of ligninsulphonic acid, and also methylcellulose, polyoxyethylene/fatty acid ester, polyoxyethylene/fatty alcohol ether, for example alkylarylpolyglycol ether, alkyl sulphonates and protein hydrolysates.
Even dispersants themselves can be processed from their solutions into readily redissolvable granulates.
It is also possible to process slurries of anionic detergents, if desired in the presence of additives of nonionic surfactants, builders, optical brighteners, of softeners and/or of scents.
The binders can be any binder (adhesive) customarily present in water-dispersible and water-soluble granulates or in oil-dispersible and oil-soluble granulates. Preference is given to the use of solutions, emulsions or latexes of natural or synthetic substances, such as methylcellulose, dextrose, sugar, starch, alginates, glycols, polyvinylpyrrolidone, ligninsulphonate, gum arabic and polyvinyl alcohol and polyvinyl acetate in water or low-boiling organic solvents, such as methanol, ethanol, butanol and methylene chloride.xe2x80x94In some cases it is even possible to use waterglass and silica sol.
Examples of preservatives which can be present in the liquid products to be sprayed in the course of the process according to the invention are 2-hydroxy-biphenyl, sorbic acid, p-hydroxybenzaldehyde, methyl p-hydroxybenzoate, benzaldehyde, benzoic acid and propyl p-hydroxybenzoate. Examples which may be mentioned of dyestuffs which can also be used as additives are inorganic pigments, such as iron oxide, titania and blue ferrocyanide, and organic dyestuffs, such as alizarin, azo and metal-phthalocyanine dyestuffs.
If, in the course of the process according to the invention, use is made of active components and binders which are present in the form of solids at room temperature, it is necessary to introduce these active components or binders into the fluidised bed in the form of a melt, of a solution or a suspension. To prepare solutions or suspensions of such active components or binders any customary inert organic solvent and water can be used. Organic solvents which can be preferably used for this purpose are alcohols, such as ethanol and glycol, and also aliphatic and aromatic, optionally halogenated hydrocarbons, such as ligroin, hexane, petrol, benzene, toluene, xylene, methylene chloride, carbon tetrachloride and chlorobenzene, and also ethers, such as dioxane, tetrahydrofuran and anisole, and also ketones, such as acetone, methyl ethyl ketone and cyclohexanone, and furthermore highly polar solvents, such as hexamethylphosphoramide, acetonitrile, dimethylformamide and dimethyl sulphoxide. Particular preference is given to the use of water.
The solids content of the liquid products sprayed into the fluidised bed in the course of the process according to the invention can be varied within a relatively wide range. In general the solids content on the use of suspensions (slurries) is between 5 and 75% by weight, preferably between 10 and 65% by weight.
The liquid products to be sprayed are prepared in conventional manner by mixing the constituents in the desired mixing ratios and, if appropriate, subsequently heating the resulting mixtures.
The granulation can be carried out in air or in inert gases, such as, for example, nitrogen. In the process according to the invention, the granulation can be started up in a fluidised-bed apparatus which already contains starting granulate. However, it is also possible to start the granulation in an empty apparatus. In this case the fluidised-bed granulation according to the invention starts off as a spray-drying. Through gradual build-up of the fluidised bed it then leads to a packed bed wherein the granulates reach the desired size and are discharged. If the products used tend to form deposits on the walls of the apparatus, it is advantageous to start up the process by presenting starting granulate. The effect of this measure is substantially to avoid possible spraying against the walls.
The liquid product to be granulated is introduced into the fluidised bed in the course of the process according to the invention by means of spray nozzles. It is particularly advantageous to use two-material nozzles.
The atomisation gas used can be any gas which is inert under the operating conditions. Preference is given to the use of air or inert gases, such as, for example, nitrogen. The amount of atomisation gas can be varied within a relatively wide range, and it generally depends on the dimensions of the apparatus and on the nature and the amount of the product to be sprayed in. In general, the amounts of atomisation gas used are, relative to the product used, 0.1 kg of gas/kg of feed to 10 kg of gas/kg of feed, preferably 0.5 kg of gas/kg of feed to 5 kg of gas/kg of feed. The temperature of the atomisation gas stream can likewise be varied within a relatively wide range. The atomisation gas temperatures used are generally between 0xc2x0 C. and 250xc2x0 C., preferably between 20xc2x0 C. and 200xc2x0 C.
The proportions of fine material which escape from the fluidised bed in the off-gas are separated off and are returned into the fluidised bed as nuclei for the granulate formation process. The way the fine material is returned can be internal or external. In the internal return of fine material, the dust is separated off at a filter placed directly on the fluidised bed and is transported back into the fluidised bed by means of cleaning-off pulses. In the external return of fine material, the dust is separated from the off-gas outside the granulator. To separate the escaping proportions of fine material, any apparatus customarily used for such purposes can be used. In a particularly preferred embodiment, the fine material is separated off by means of a cyclone or a dust filter. The separated-off fine material is transported back into the spray zone of the fluidised bed. This return transport is preferably effected pneumatically. The drive gas used can be any customary gas which is inert under the operating conditions. Preference is given to the use of air and inert gases, such as, for example, nitrogen. The amount of drive gas can be varied within a relatively wide range; it depends in general on the dimensions of the apparatus and on the escaping amount of fine material. The amounts of drive gas used are in general 0.01 kg of gas per kg of fine material to 2 kg of gas per kg of fine material, preferably 0.1 to 1 kg of gas per kg of fine material. The temperature of the drive gas stream can likewise be varied within a relatively wide range. The temperatures used are generally between 20xc2x0 C. and 350xc2x0 C., preferably between 30xc2x0 C. and 300xc2x0 C.
The granulation process in the fluidised bed in the process according to the invention is maintained solely by the sprayed-in rate of the liquid product to be granulated and the intensity of the classifying gas stream. No additional nuclei are supplied from the outside. The classifying gas used can be any customary gas which is inert under the operating conditions. Preference is given to the use of air and inert gases, such as, for example, nitrogen. The amount of classifying gas can be varied within a relatively wide range; it depends on the dimensions of the apparatus and the grain size and the mass flow of the granulates to be discharged. The classifying gas amounts used are in general between 0.2 kg of gas per kg of granulate and 5 kg of gas per kg of granulate, preferably between 0.4 and 2 kg of gas per kg of granulate. The temperature of the classifying gas stream can likewise be varied within a relatively wide range. The classifying gas temperatures used are in general between 20xc2x0 C. and 350xc2x0 C., preferably between 30xc2x0 C. and 300xc2x0 C.
The classifying gas speed depends on the grain size and the density of the granulate to be discharged. The classifying gas speeds used are in general between 0.5 and 15 m/sec, preferably between 1 and 5 m/sec.
The completed granulate is discharged in the course of the process according to the invention by way of one or several countercurrent.gravity classifiers. This type of discharging element can be any customary classifier which works by the principle of countercurrent gravity classification. If a particularly narrow grain distribution is desired, a zig-zag classifier is used as a specific embodiment.xe2x80x94In order to keep the amount of classifying gas as low as possible, for reasons of energy, in carrying out the process according to the invention preference is given to the use of a classifier having a zig-zag cross-section (=zig-zag classifier) where the gap length and hence the classifier cross-section can be set by means of bars which are connected to one another in the manner of a comb, which are adapted to the zig-zag cross-section and which are slideable at right angles to the axis of the classifier. In a preferred embodiment, the classifier contains an adjusting means for the bars which is connected to a regulating device which regulates the classifying gas stream in such a way that the flow velocity in the classifier remains constant despite the variable cross-section.
The process according to the invention is generally carried out under atmospheric pressure. However, it is also possible to work under superatmospheric pressure or reduced pressure. The output point of the countercurrent gravity classifier is generally kept under atmospheric pressure. Said atmospheric pressure is obtained by connecting to the classifier, in between the off-air fan and the output point, a pressure regulator which adjusts the off-air fan or a butterfly valve or an analogous device and which constantly adapts the pressure at the output point of the classifier to the ambient pressure. If no atmospheric pressure prevails at the output point of the classifier, it is necessary to install locks for maintaining the desired pressure.